Normally, a vehicle is equipped with a plurality of control devices that are for performing electric power control and/or driving system control of the whole of the vehicle while controlling states of secondary batteries for use in power sources. More particularly, a hybrid vehicle is equipped with a DC-DC converter which is connected between a high-voltage system power source including a high-voltage system secondary battery that supplies electric power to a high-voltage system load and a low-voltage system power source including a low-voltage system secondary battery to which a low-voltage system load is connected, and performs control of transmission/reception of electric power on the basis of information of charge states and load states of the high-voltage system secondary battery and the low-voltage system secondary battery.
So, in order to efficiently perform the control of transmission/reception of electric power between the low-voltage system secondary battery and the high-voltage system secondary battery, for example, a DC-DC converter of Patent Document 1 is provided with a battery monitoring device that monitors voltage states, charge states, and the like of secondary batteries by sensors, the battery monitoring device being provided inside the DC-DC converter, in the transmission/reception of electric power between the secondary batteries, whose role is supported by the DC-DC converter. Output voltage of the DC-DC converter is controlled to be a constant voltage by pulse width modulation (PWM) control on the basis of information from the battery monitoring device.
Furthermore, when AD converters that convert analog signals from various types of sensors into digital signals have failed, a state detection device of a secondary battery, whose voltage source is the secondary battery, calculates a state of charge of the secondary battery on the basis of an unauthorized output from the AD converters; and accordingly, the state of charge of the secondary battery cannot be accurately calculated. Thus, in the case of being used continuously in a state where the AD converters have failed, it may not be possible to start an engine due to deterioration and/or over-discharge of the secondary battery. Further, it becomes an output voltage of the DC-DC converter which is different from control intention, and accordingly it may not be possible to efficiently transmit/receive electric power between a high-voltage system secondary battery and a low-voltage system secondary battery. As countermeasures against this, for example, in a failure diagnosis method that is for a state detection device of a secondary battery of Patent Document 2, the state detection device of the secondary battery, which has a plurality of AD converters that convert analog values from sensors that detect states of the secondary battery into digital values, includes a calculation unit that mutually compares failure diagnosis voltages to be inputted to the plurality of AD converters with output values of the plurality of AD converters to perform failure determination of the AD converters. This proposes a method of accurately performing failure diagnosis of the AD converters even when the failure diagnosis voltages fluctuate.